1. Field Of The Invention
This invention relates to an engine oil having improved fuel economy properties.
2. Description Of The Related Art
The goal of increasing gasoline mileage of automobiles under the Federal Corporate Average Fuel Economy (CAFE) standards has resulted in increased interest in improving the fuel economy performance of engine oils. Increasing the fuel economy performance of engine oils has resulted in greater emphasis on friction modifiers.
While the majority of moving parts in an internal combustion engine are in a state of hydrodynamic lubrication, some sliding parts such as pistons and valve trains are in a boundary lubrication state. In order to provide wear resistance caused by friction in the boundary lubrication state, it is necessary to provide the engine oil with additives to reduce wear. For many years, zinc dialkyldithiophosphates ("ZDDP") have been a standard antiwear additive. While ZDDP is a good antiwear agent, it has negative impacts on fuel economy. Thus it is usually necessary to include a friction modifier for fuel economy purposes. Both antiwear and friction modifiers function through adsorption on the sliding metal surface and may interfere with each other's respective functions.
U.S. Pat. No. 5,356,547 describes an engine oil having a low coefficient of friction from an early operating stage. European Patent Application 0562172 A1 describes an engine oil having low friction properties from an early operating stage and continuing under longer periods of use.
European published application EP 0562172 describes an engine oil having low friction properties and containing a boron derivative of an alkenylsuccinimide, an alkaline earth metal salt of salicylic acid and a molybdenum dithiophosphate or dithiocarbamate.
It would be desirable to have an engine oil with improved fuel economy and fuel economy retention properties.